The method for total reflection X-ray spectroscopy is adapted for irradiating primary X-rays to a measured sample, by which an element present in the measured sample emits secondary X-rays referred to as characteristic X-rays, and conducting elemental analysis for the sample by using the characteristic X-ray, and this has been known long seen.
Since the wavelength of the characteristic X-rays is inherent to the element which is present, an element present in the sample can be analyzed qualitatively based on the wavelength of each of characteristic X-rays appearing in the characteristic X-rays of the measured sample. Further, since each characteristic X-ray intensity reflects the existing quantity of the element, it is also possible to recognize the existing quantity of the element based on the analysis of the characteristic X-ray spectrum. In this case, it is generally required that a standard sample having a known quantity of an analyzed element is provided and a relation between the quantity and the characteristic X-ray intensity is determined previously.
What should be noted in the method for X-ray fluorescence spectroscopy is that not only the characteristic X-rays appears in the characteristic X-ray spectrum for the sample irradiated with primary X-rays but also scattered X-rays scattered in the inside of the sample is also included, and the scattered X-ray intensity constitutes a major factor for determining a detection limit of the X-ray fluorescence spectroscopy. That is, if the element present in the sample emits characteristic X-rays only at an intensity equal with or lower than the fluctuation of the scattered X-rays, the element can not be analyzed quantitatively.
By the way, while the X-ray fluorescence spectroscopy is applicable also to the analysis of microelements localized only at the surface of a semiconductor or the like, a problem due to scattered X-rays is caused in this case. That is, since the analyzed element is present only on the surface of the sample, the characteristic X-rays thereof are also emitted only from the surface, whereas scattered X-rays are emitted also from the inside of the sample. Accordingly, for the intensity of both of them, the latter is outstandingly great and no sufficient detection limit can be obtained.
As an analyzing method for overcoming the problem, it has been known total reflection X-ray fluorescence spectroscopy of irradiating primary X-rays to a sample at a smaller angle than a critical total reflection angle and conducting elemental analysis for the surface of the sample by using the characteristic X-rays excited therewith. In this total reflection X-ray fluorescence spectroscopy, the intruding depth of the primary X-rays into the sample is extremely shallow and it is considered to be an order of several nm, for example, in single crystals of silicon. Therefore, the scattered X-ray intensity emitted in the sample is reduced greatly, and an analyzed element present only on the surface of the sample can be analyzed at a low detection limit. The feature of the method for total reflection X-ray fluorescence spectroscopy, as can be seen from the principle thereof, resides in that the characteristic X-rays for the analyzed element on the surface of the sample are excited substantially only by the primary X-rays.
For obtaining an analysis value by the total reflection X-ray fluorescence spectroscopy, calibration with a standard sample is required like that in the case of a usual X-ray fluorescence spectroscopy. In the case of the total reflection X-ray fluorescence spectroscopy, since the object is measurement, particularly, for the quantity of the analyzed element on the surface of the sample calibration is conducted by using a standard sample having a known quantity of the analyzed element on the surface. In the present specification, the characteristic X-ray intensity for the known quantity of the analyzed element determined by using a standard sample is hereinafter referred to as a calibration coefficient.
However, the existent total reflection X-ray fluorescence spectroscopy described above involves a not yet solved subject as described below.
That is, while it is required to irradiate the primary X-rays at an angle smaller than a critical total reflection angle to the sample in the total reflection X-ray fluorescence spectroscopy in view of the principle thereof, it is pre-conditioned that the sample surface is an ideal smooth surface.
For instance, a silicon wafer can be regarded as an ideal smooth surface so long as it is within a range of a beam diameter of X-rays employed usually. However, the object for the analysis of the total reflection X-ray fluorescent spectroscopy is not always restricted to a sample having such an ideal smooth surface.
Many thin film samples, for example, of polysilicon often have irregularities about from several tens to several hundreds nm. If such irregularities are present on the surface, even when primary X-rays are irradiated at an angle smaller than the critical total reflection angle to an averaged sample surface (for example, an envelope plane for the irregularities on the surface), there is present a portion irradiated with an angle greater than the critical total reflection angle when observed locally, and primary X-rays intrude into the sample through such a portion to increase the scattered X-ray intensity.
As a result, an undesired situation as described below will be caused in the total reflection X-ray fluorescence spectroscopy for a sample having irregularities. That is, assuming that a standard sample for obtaining an analysis value has a substantially ideal smooth surface, even when the standard sample and the measured sample are analyzed under identical conditions, the intensity of the scattered X-rays emitted from the inside of the sample is much more greater in the later.
Therefore, the characteristic X-rays from the analyzed element present on the surface of the measured sample are excited not only by the primary X-rays but excited also by the scattered X-rays from the inside of the sample. Accordingly, no accurate analysis value can be obtained if calibration is conducted by using the characteristic X-ray intensity for the standard sample, as it is, for which scattered X-rays from the inside of the sample is negligible substantially.
For avoiding such a situation, it may be considered to prepare a standard sample itself with a sample having the same irregularities as those of the measured sample. However, this method involves the following not yet solved subjects.
At first, it is difficult to form identical irregularities both for the standard sample and the measured sample. Identical irregularity means here that the scattered X-ray intensity caused by the irregularities are identical. That is, irregularities for both of them can not be equal unless a standard sample emitting scattered X-days identical with those of the measured sample and it is generally difficult to prepare such a standard sample.
Secondly, in a case of analyzing samples having various irregularities, preparation of standard samples corresponding to respective irregularities requires an operation of depositing a known quantity of an analyzed element to the surface of standard sample and calibration to determine whether the quantity of the analyzed element is surely equal with a predetermined quantity, which is time and cost consuming.